Various apparatus and methods for fast pyrolysis of biomass such as wood, bark, grasses and legumes to produce bio oil and char are well known. While fast pyrolysis is related to traditional methods and apparatus for making charcoal, fast pyrolysis is a modern process in which the heating is carefully controlled to give high yields of bio oil rather than charcoal. Wood or other biomass is heated as rapidly as possible to temperatures in the range of 350° C. to 550° C. causing decomposition reactions that produce gases, oil vapors and charcoal or char. The vapors are cooled and condensed as rapidly as possible forming bio oil. The bio oil can be used as a heating fuel or upgraded for use as a motor fuel. Slow pyrolysis and slow vapor cooling and condensation such as occurs in the conventional charcoal manufacturing yields smaller amounts of oil and greater amounts of charcoal and non-condensable gases.
Many different reactor designs have been studied in the development of fast pyrolysis during the past thirty years. A common design approach is a reactor enabling the rapid mixing of a small quantity of finely ground biomass particles with large quantities of heated sand particles. Transported beds, circulating fluid beds, bubbling fluidized beds, auger mixing and rotating cones have been used. In a recent variation of this approach heated metal particles replace sand.
A list of reported fast pyrolysis reactors worldwide, both industrial and research, was presented in a recent publication, International Energy Agency Bioenergy Agreement Task 34, Pyrolysis, PyNe Newsletter #27, June 2010. Of the 44 industrial reactors, 12 are classified as fluid or bubbling bed, 9 are classified as circulating or transported bed, and 9 are classified as auger or screw. No rotating reactors are listed. The three major classifications are described in the following to provide a basis for evaluating the advantages of the present invention.
The largest existing fast pyrolysis reactor is the 8,000 kg/hr bubbling bed unit constructed in 2007 by Dynamotive at Guelph, Ontario, Canada. U.S. Pat. No. 5,728,271 describes the apparatus and method of operating this reactor. Biomass solids are dried to less than 10% moisture and ground to less than 3 mm in the shortest dimension. Inert solid particles, typically silica sand, smaller than 20 mesh (0.84 mm) is specified as the fluidized bed heat transfer material. The fluidizing gas is recycled pyrolysis gas, the flow rate being such that the mass ratio of fluidizing gas to biomass is less than 2:1. The superficial velocity of the fluidizing gas through the bed should be in the range 10-80 cm/sec and its residence time in the reactor should be in the range of 2-25 seconds. The patent further specifies that the fluidized bed is heated indirectly by hot gases and an embodiment is described with the hot gases flowing through tubes passing through the bed. The hot gases are created by combustion of recycled pyrolysis gas, char or a supplemental fuel. The biomass is fed by an auger into the bed above the heat exchanger. Pyrolysis vapor, gases, char and sand dust formed by attrition of the sand particles are swept out of the bed by the fluidizing gases and into a cyclone separator located at the top of the reactor. Char and sand dust are separated and flow downward from the cyclone through the hot gas combustor to a screw conveyor for removal to a collection bin. The pyrolysis vapor and gases are carried from the cyclone separator to a condenser and a demister.
A 7.8 cm diameter laboratory bubbling bed reactor is described by Boateng, et al, Bench-Scale Fluidized-Bed Pyrolysis of Switchgrass for Bio-Oil Production, Ind. Eng. Chem. Res. 2007, 46, 1891-1897. This reactor is operated with a fluidizing gas superficial velocity of 65 cm/sec. It has a biomass pyrolysis capacity of 2.5 kg/hr. The data reported imply a mass velocity or scaling factor of 520 kg/hr/m2.
A 2005 U.S. Department of Energy publication reported that the fluidized bed reactor of the Thermochemical Users Facility at the National Renewable Energy Laboratory is a 1.8 m high cylindrical vessel 20 cm diameter in the lower (fluidization) zone, expanded to 36 cm diameter in the freeboard section. It is equipped with a perforated gas distribution plate and an internal cyclone to retain entrained bed media, typically sand. The reactor is heated electrically and can operate at temperatures up to 700° C. at a throughput of 15-20 kg/h of biomass. The NREL data imply a mass velocity of 480-640 kg/hr/m2.
The equipment required for operating the fluidized bed reactor in addition to the biomass preparation and feeding equipment include; (1) a vertical tubular reactor containing the fluidized sand bed and related freeboard volume, (2) a tank to receive and store replacement sand, (3) an elevator to transfer replacement sand into the reactor, (4) a tubular heat exchanger or other means transferring heat into the sand bed, (5) a burner to supply heat to the exchanger, (6) an air blower for the burner, (7) a cyclone separator for separating char and sand dust from the pyrolysis vapor and gases, (8) a blower and pipeline for recycling pyrolysis gases to the fluid bed reactor.
PyNe Newsletter #27 reports that the 4,000 kg/hr unit built by Ensyn Corporation in Renfrew, Ontario, is the largest existing circulating bed fast pyrolysis unit. The Ensyn technology is covered by U.S. Pat. Nos. 5,961,786, 6,485,841 B1 and patent application publication US 2009/0266380 A1, October 2009. A vertical tubular reactor is specified with a base section where the biomass particles are rapidly mixed with hot inorganic particles, typically sand, and recycled gas. The size of the biomass particles is typically less than 6 mm and the size of the inorganic particles ranges from 0.04 and 0.4 mm. The ratio of the mass of the sand to the mass of the biomass particles is specified to be greater than 12:1. The recycle gas/biomass ratio is not specified, however a recent Pacific Northwest National Laboratory report (S. Jones, et al, Production of Gasoline and Diesel from Biomass via Fast Pyrolysis, Hydrotreating and Hydrocracking: A Design Case, PNNL-18284 Rev 1, DOE Contract DE-AC05-76RL01830, February, 2009) indicates this ratio is near 0.9. The mixed particles and recycle gas flow upward at superficial velocities greater than 2 meters per second and the pyrolysis reactions are completed in reactor residence times less than 2 seconds. Reactor biomass throughput rates are specified to be greater than 3,910 kg/hr/m2.
Sand, recycle gas and pyrolysis products flow out of the top of the reactor into a cyclone separator or multiple cyclone separators where the sand and pyrolysis char are separated from the gas and vapor. Char is combusted in a fluidized bed to heat the sand particles. Make-up sand to replace attrition is fed with recycle sand to the circulating bed.
The equipment required for operation of the circulating bed pyrolysis reactor in addition to the biomass preparation and feeding equipment include; (1) a vertical tubular reactor for mixing biomass particles, hot sand particles and recycled gas, (2) a tank to receive and store replacement sand, (3) an elevator to transfer replacement sand into the sand heater, (5) a recycle gas blower to convey recycled gas to the pyrolysis reactor, (5) a cyclone for separating coarse char and sand from gas and vapor, (6) a cyclone for separating fine char and sand dust from the gas and vapor, (7) a sand heating vessel containing the fluidized sand and char particles, (8) a sand heater air blower for fluidizing the sand and combusting the char particles, (9) a cyclone for separating fine char and sand dust from the sand reheater flue gases, (10) a thermal oxidizer vessel for char fines, (11) a thermal oxidizer air blower, (12) a thermal oxidizer bag house.
A recent publication by R. C. Brown and J. Holmgren (Fast Pyrolysis and Bio-Oil Upgrading, Advanced Biofuels USA, Dec. 29, 2009) discussed the relative merits of various reactors and concluded auger mix reactors offered lower inert gas requirements and lower complexity than fluid or circulated beds. PyNe Newsletter #27 reports a demonstration scale auger mix pyrolysis reactor being developed by the Karlsruhe Institute of Technology (KIT) was commissioned in Germany in 2008. The KIT reactor has a design capacity of 12 tonnes per day of dried biomass such as straw. The design specifies sand to biomass ratios in the range of 5:1 to 10:1. Therefore, from 60 to 120 tonnes per day of sand heated to 550-600 C is fed to this reactor to rapidly heat 12 tonnes of straw to 500 C. The biomass and char are mixed in a horizontal tubular reactor by twin screws. The straw particles rapidly decompose to oil vapor, non-condensable gases and char. The residence time of the vapors and gases in the reactor is approximately 3 seconds.
Replacement sand is added to the biomass feed stream to compensate for dust forming attrition of the sand particles as they flow through the reactor and recirculation system. The sand attrition rate is stated to be about 1% of the biomass feed rate. Vapors, gases, sand dust and char flow out of the top of the reactor and into a cyclone above the reactor where fine char and sand dust are separated and sent to a screw cooler and collection vessel. The oil vapors and non-condensable gases flow out of the cyclone into two condensers in series. Sand and char flow out of the bottom of the reactor into a surge tank and then to a lift pipe where they are picked up by hot air and combustion gases from a pyrolysis gas burner. The lift pipe carries the sand and char up to a separation tank above the reactor. Char particles are combusted in the lift pipe to supplement the pyrolysis gas. The combustion gases flow out of the sand separator tank and into cyclone separator, a filter and then into a flare stack.
The equipment required to operate the auger-mix pyrolysis reactor in addition to the biomass preparation and feeding equipment include: (1) a tubular reactor encasing twin mixing augers, (2) a tank to receive and store replacement sand, (3) an elevator to transfer replacement sand into the biomass feed line, (4) a heated sand separation tank located above the reactor, (5) a sand discharge surge tank located below the reactor, (6) a solids transfer pipe between the sand discharge surge tank and lift pipe, (7) a lift pipe to carry the sand back to the separation tank above the reactor, (8) a sand heating burner to combust pyrolysis gas and provide hot gas for the lift pipe, (9) an air blower for the sand heating burner, (10) a cyclone for removing char and sand particles from the combustion gases, and (11) a filter downstream of the cyclone.